Lighting solutions using diodes are becoming more prevalent as the manufacture of powerful and bright diodes is becoming more economical. A diode lighting arrangement can comprise a single light-emitting diode, or several light-emitting diodes connected in series. The diode(s) should be protected from electrostatic discharge, since the current surge passing through a diode (in either direction) as a result of electrostatic discharge can severely damage the diode. For this reason, prior art protection circuits include an electrostatic discharge (ESD) diode, usually a pair of zener diodes arranged back-to-back, connected in parallel with the light-emitting diode(s). Depending on the surroundings in which the diode lighting arrangement is operated, high voltages in the region of several tens of kilovolts can build up. For an encapsulated device, the light-emitting diodes and the ESD diode are enclosed in a plastic dome or package, so that an electrostatic discharge can only unload into a terminal of the packaged device, and the current can safely discharge through the ESD diode.
In a high-power LED arrangement, where the brightness or luminance of the light source is essential as e.g. in automotive headlamp applications the LEDs are usually not encapsulated with such a dome or plastic cover. A dome can improve the outcoupling efficiency, but usually magnifies the source and thus reduces luminance. Instead, the flat LEDs are arranged side by side on a substrate, and can either be left exposed, or can be coated with a material that serves to adjust the colour of the emitted light. White light is typically generated by converting part of the blue light emitted from an LED into yellow light. The mixing of the blue and yellow light results in a white colour. The series connection of two or more such ‘exposed’ light-emitting diodes is generally made by metalized areas extending from the positive terminal of one diode to the negative terminal of the next diode in series. In such a ‘multi-junction’ or ‘multi-die’ arrangement, the light-emitting diode dies can be vulnerable to ESD damage, even when an ESD diode is used, since a surge might discharge at an interconnect between two neighbouring dies, so that a high current flows through one or more of the exposed dies. Even if the dies are surrounded by a filler material (which can be used to collect any light escaping at the edges of a phosphor coating applied to the emitting surface of the dies), such a filler may not extend to completely cover the edges of the flat metalized interconnect surfaces. Even a small exposed area can serve as a point at which electrostatic discharge can unload and damage or destroy one or more LEDs. Electrostatic charge can build up, for example, on the inside of a plastic or glass outer cover enclosing such a diode lighting arrangement from a distance, for example the outer cover of a vehicle headlight or rear light that encloses the diode lighting arrangement, a ballast, any secondary optics, etc. The electrostatic charge, which can easily reach 10 kV or more, might discharge into one of the interconnects, and the resulting high current can damage or destroy one or more of the light-emitting diodes. Therefore, these interconnects are a weak point of such a lighting arrangement. One possible solution might be to coat the entire interior surface of such a plastic or glass housing with a transparent conductive material such as indium tin oxide, so that electrostatic discharge cannot build up. However, such a coating is expensive, so that this solution is not particularly attractive. An alternative solution might be to connect the interior surface of the (uncoated) plastic housing to a lower potential such as ground, for example by means of a flat metal strip connected between the cover and the ESD diode of the lighting arrangement or another current sink of the ballast, so that any electrostatic charge would be diverted safely away from the lighting arrangement. However, particularly in an automotive lighting application, it can be complicated and costly to ensure that the different components are connected to ground, since each component (lighting arrangement, ballast, outer cover) may be sourced from a different supplier. Such a complex solution could add considerably to the overall cost of a lighting arrangement.
Therefore, it is an object of the invention to provide a straightforward and economical way of protecting a diode lighting arrangement from electrostatic discharge.